1. Field of the Invention
The present invention relates to a light-emitting device having high light-extraction efficiency.
2. Description of the Related Art
There are known light-emitting devices of various configurations. An example of such a known light-emitting device is illustrated in the sectional view in FIG. 6. The light emitting device 1000, which is illustrated in FIG. 6, includes a front panel 1001, a light-emitting layer 1002, and a transparent electrode 1003, which is part of an excitation source for exciting the light-emitting layer 1002. The front panel 1001 is made of a transmissive medium that transmits visible light, for example, glass or plastic. The excitation source includes, for example, an electron-emitting device 1005, which opposes the front panel 1001, and the transparent electrode 1003, which is disposed on the front panel 1001. With such a configuration, electrons emitted as a result of applying an electric field to the electron-emitting device 1005 are accelerated in the transparent electrode 1003, which is disposed on the front panel 1001, and are incident on the light-emitting layer 1002, causing light to be generated. The light generated at the light-emitting layer 1002 is transmitted through the front panel 1001 and is extracted to the outside as output light 1004. The proportion of the output light 1004 extracted to the outside to the light generated at the light-emitting layer 1002 is known as “light-extraction efficiency.”
One cause of a reduction in light-extraction efficiency in the emitting device 1000 is loss caused by total internal reflection at the boundary of the front panel 1001 and the transparent electrode 1003 of the excitation source or at the boundary of the light-emitting layer 1002 and the transparent electrode 1003 of the excitation source. Specifically, it is known that when light is transmitted through a boundary between a high refractive index medium to a low refractive index medium, light that is transmitted at an angle larger than a critical angle of incidence is totally reflected and trapped inside the high refractive index medium. The reflected and trapped light is not extracted into the low refractive index medium, instead it is transmitted through the high refractive index medium, and is lost.
To reduce total reflection loss and improve light-extraction efficiency, U.S. Pat. No. 6,476,550 describes a configuration in which a fine structure is interposed between members having different refractive indexes, as illustrated in FIG. 7. A light-emitting device 1100, which is illustrated in FIG. 7, includes a front panel 1101, a light-emitting layer 1102, a transparent electrode 1103, an electrode layer 1104, and a fine structure 1105, which is interposed between the front panel 1101 and the transparent electrode 1103. The fine structure 1105 includes multiple media having different refractive indexes and has a refractive index distribution with a cycle similar to the wavelength of light. In this manner, light that is generated inside the light-emitting layer 1102 and transmitted at an angle larger than the critical angle is converted to light that is transmitted at an angle equal to or smaller than the critical angle by diffraction, thus increasing the amount of light 1106 extracted to the outside.
With the procedures described in U.S. Pat. No. 6,476,550, there is a need for improvement in the light-extraction efficiency. In FIG. 7, part of the light emitted from the light-emitting layer 1102 and transmitted through the fine structure 1105 is transmitted and diffracted, and the remaining light becomes 0-order transmitted light. When light having a small incident angle is incident on the fine structure 1105, part of the light is diffracted as diffracted light 1107, which is transmitted at an angle larger than the critical angle. The diffracted light 1107 is totally reflected at the boundary and lost. When light having a large incident angle is incident on the fine structure 1105, non-diffracted light, i.e., 0-order transmitted light 1108, is transmitted at an angle larger than the critical angle at the boundary of the front panel 1101 and the outside, is totally reflected at the boundary, trapped inside the front panel 1101, and is lost. In other words, with known procedures, increasing the diffraction efficiency of the fine structure 1105 increases the amount of diffracted light 1107, and decreasing the diffraction efficiency increases the amount of the transmitted light 1108. In this way, the fine structure 1105 limits the light extracted to the outside to light incident on the fine structure 1105 at a specific angle, and the light-extraction efficiency is not sufficiently improved.